In the melt spinning of molten polymers to produce filaments increased efficiency is nearly always a worthwhile goal. One manner of increasing the efficiency of a melt spinning process is to increase the number of fibers which can be produced during a given time from a single piece of melt spinning machinery. In furtherance of this goal, spinneret plates providing for an increased number of filaments to be extruded therethrough is of value.
Another consideration in melt spinning operations is the cross section of the extruded filament. Fibers having novel cross sections may be useful for a variety of different purposes, some of which purposes are readily apparent from the unique cross section and others which remain to be discovered. Fibers of new deniers are also invaluable. Furthermore, new combinations of deniers and cross-sections can result in commercially interesting fibers.
For the present purposes, the term "counterbore" refers to the upstream bore in a spinneret plate and its upstream orifice. The term "capillary" refers to the downstream orifice in a spinneret plate and its downstream orifice.
The following patents exemplify efforts to modify the melt spinning process and the characteristics of the resulting melt spun filament. In general, the characteristics of only a few classes of spinneret capillaries have been determined with respect to kneeing and for only a few general classes of capillary shapes. Smith U.S. Pat. No. 2,838,364 discloses that cellulosic fibers may be spun in a manner to produce filaments of hollow cross section through a spinneret having a plurality of counterbores each in the shape of a sector of a circle.
Imobersteg et al. U.S. Pat. No. 3,405,424 discloses a spinneret for manufacturing hollow synthetic fibers from counterbore groups having at least two laterally opposing star-shaped capillaries. X and Y star shapes are disclosed. Sometimes high pressures on the upstream side of the spinneret plate forces the legs of the star-shaped capillary apart.
Shemdin, U.S. Pat. No. 3,652,753 and Shemdin U.S. Pat. No. 3,860,679, describe a formula for predicting the appropriate capillary shape to eliminate the phenomenon of kneeing. Kneeing is defined as "when the line of flow of the filament is bent out of the vertical back toward the spinneret face at an angle with respect to the perpendicular to the spinneret face." Kneeing may be so severe that the line of flow actually bends back and touches the spinneret face or it may be only sufficient to cause two or more adjacent filaments to touch and coalesce. The capillaries are generally T-shaped.
Paliyenko et al. U.S. Pat. No. 3,734,993 teaches that the effect of kneeing in T-shaped capillaries is reduced if the stems of the T's are arranged such that each stem extends perpendicularly outward (relative to the spinneret die plate) from the cross bar.
Phillips U.S. Pat. No. 3,981,948 discloses that kneeing may be used to coalesce individual molten streams. Phillips extrudes individual molten streams through non-round orifices which are dimensioned according to a specified formula. The formula assures that the coordinates of the centroid of the square of the velocity profile of the extruding material in the plane perpendicular to the axis of the capillary and the coordinates of the centroid of velocity profile of the extruding material in the plane perpendicular to the axis of the capillary are non-coincident.
Conversely, Phillips U.S. Pat. No. 4,142,850, describes that certain non-round spinneret capillaries eliminate the kneeing of extruding filaments. The patent applies a formula for configuring the orifices by using the centroid of the square of the velocity profile of the extruding material and the centroid of the velocity profile of the extruding material. When these two parameters are co-incident at each capillary exit, the extruding filaments should not knee.
In addition to the foregoing, there are various patents showing that rounded capillaries can be spaced or configured such that their respective extruded streams will merge prior to solidification. Hodge U.S. Pat. No. 3,924,988 shows a spinneret provided with a group of capillaries each defining an arcuate segment and having inwardly tapered enlargements. The particular structure causes a velocity differential to occur in the polymer flow that favors coalescence at the tapered portions to form a single round or rounded hollow filament.
Gintis et al. U.S. Pat. No. 4,407,889 shows a method for preparing splittable hollow filaments. These filaments have longitudinal grooves and ridges that are readily split along the grooves. The spinneret used to produce these fibers includes a group of capillaries arranged so hat the molten streams issuing therefrom each bulge as they leave the face of the spinneret, causing the streams to coalesce and form the desired hollow filament.
In Yu et al. U.S. Pat. No. 4,325,765, high denier non-round filaments are produced by extruding a molten polymer (polyester) through adjacent orifices which are spaced such that the extruded streams merge prior to solidification. This patent addresses the stated problem that markedly non-round cross-sectional filaments having deniers of at least 10 could not be successfully melt spun from polyester polymers at high speeds using the techniques known at that time.
In contrast to patents showing the goal of melt stream coalescence, there are also patents directed to spinnerets designed to allow filaments to be extruded in high density without coalescence. One such patent is Pfeiffer et al. U.S. Pat. No. 4,318,680 which shows a spinneret plate having multiple capillaries per counter-bore that effectively melt spins fusion melts of acrylonitrile polymer and water without coalescence. The patent is concerned with round cross-sections.
There remains a need for a manner of producing fibers with non-round cross-sections in high density and without coalescence. This goal has, to Applicant's knowledge, been elusive.